152 Dr. T. Williams 07i the Mechanism of Aquatic 



primary also in a determinate manner, separated by tolerably 

 uniform distances, and running in tolerably orderly directions, 

 so that they leave between them spaces which manifest a ten- 

 dency to symmetry of outline and regularity of arese. 



The ultimate vessels (fig. 6 c, c) undoubtedly obey a similar 

 method of distribution. If a spot be taken for inspection in 

 which they are densely present, the vessels will be found to lie in 

 parallel columns (as represented in PL XI. fig. 6, which has been 

 drawn with great care from the actual object). In other places, 

 especially towards the circumferences of the roof, they exhibit a 

 more sparse and less regular distribution. If such portions of 

 this vascular membrane were folded in the " ridge and gutter'^ 

 fashion, taking care that the secondary vessels coincided with 

 the borders of the folds, a branchia would be formed whose 

 structure would conform with the laminose principle. It may 

 conversely be said, that the lung of the air-breathing Gasteropod 

 is nothing but the branchia of the water-breathing Gasteropod, 

 having the laminae of the gill so unfolded as to form a straight 

 sheet. This comparison is really not too far-sought. In both 

 instances the ultimate vessels present a uniform cylindrical cha- 

 racter, seldom intercommunicating : such a comparison, how- 

 ever, is quite untenable in the case of Limax (fig. 2). Here 

 the plexus is arboriform and irregular, circumscribing arese of 

 various sizes and figures. Like that of Limax, the lining of this 

 cavity in Helix is undoubtedly ciliated at various parts. It is 

 only possible to detect the presence of cilia along the lines of the 

 larger vessels, and here and there in creases in their vicinities. 



From the author's observations, it is probable that the spaces 

 between the larger vessels, and coinciding with the capillary 

 arese, are devoid of ciliated epithelium. Nor is it easy to sepa- 

 rate the layer of epithelium by which these parts are covered, 

 from the elements of which the coats of the vessels themselves 

 are composed. 



In Helix the coats of the pulmonary vessels (fig. 3 h) are con- 

 siderably less calcified, less white in appearance, and more flexible 

 than those of Limax, They afford, therefore, a more favourable 

 opportunity for determining their minute structure. 



The same surprise may be expressed with respect to this lung- 

 sac as with respect to that of Limax, viz. that so limited a 

 vascular area presenting vessels so coarsely subdivided, should 

 suffice, in so bulky an animal, to supply the demands of the respi- 

 ratory function. While all other animals, even those inferior 

 to these Mollusks in serial standard, are furnished with organs 

 which involve prodigious superficies for action, how is it, it may 

 again be asked, that in these superiorly endowed Gasteropods so 

 rudely constructed an apparatus, so carelessly formed an organ 



